Francisella tularensis (Tularemia)

Francisella tularensis is a Gram-negative coccobacillus, which means that shape-wise, it’s somewhere between a spherical coccus and a rod-like bacillus.

In humans, it causes a zoonotic infection called tularemia, also called rabbit fever.

This bacteria is also considered a category A bioterrorism agent which means it is of highest concern for bioterrorism use, because of its low infectious dose and high associated mortality.

Now, Francisella tularensis has a thin peptidoglycan layer, so it doesn’t retain the crystal violet dye during Gram staining.

Instead, like any other Gram-negative bacteria, it stains pink with safranin dye.

Alright, now Francisella tularensis is non-motile, non-spore forming, facultative intracellular which means it can survive both outside and inside the cell and aerobic which means it can survive only in the presence of oxygen.

Also, it’s oxidase and urease negative which means it doesn’t produce these enzymes.

Finally, Francisella tularensis is a fastidious bacteria which requires enriched medium for growth.

And Francisella tularensis really loves cysteine, so it only grows in about 47 to 72 hours on cysteine-enriched mediums like cysteine enriched chocolate agar, BCYE and CHAB.

Cysteine enriched chocolate agar, named so for its color, actually contains cysteine and lysed red blood cells - so no chocolate products were harmed in the making of this medium.

BCYE stands for buffered charcoal yeast extract, so it contains activated charcoal, yeast extract, and L-cysteine.

On these two mediums, Francisella tularensis forms round, grey-white colonies.

Finally, CHAB is a glucose cysteine agar that contains thiamine and blood, and on CHAB, Francisella tularensis forms greenish-white, round, smooth, mucoid colonies.

Now, Francisella tularensis has a number of virulence factors, that are like assault weaponry that help it attack and destroy the host cells, and evade the immune system.

So first, Francisella tularensis is encapsulated, meaning it’s covered by a polysaccharide layer called a capsule.

And right underneath that capsule, there’s also an outer membrane, which consists of lipopolysaccharide - or LPS for short.

The most interesting part is that normally, the LPS of Gram-negative bacteria binds to a protein found on immune cells like macrophages, called Toll-like receptor 4, or TLR4 for short, which is involved in the activation of innate immune system.